Adsorption of Rhizobium meliloti to alfalfa roots: dependence on divalent cations and pH

Adsorption of Rhizobium meliloti L5-30 in low numbers to alfalfa (Medicago sativa L.) roots was dependent on the presence of divalent cations, and required neutral pH. Adsorption was proportional to Ca and/or Mg concentrations up to 1.5 mM. Ca was not substituted by Sr, Ba or Mn. Adsorption was abolished and viability decreased at pH ⩽ 6. When lowering pH, higher Ca concentrations were required to attain similar adsorption levels, indicating a marked interactive effect between Ca and H ions. Pretreatment of the roots with Ca and low pH did not affect subsequent adsorption of the bacteria. However, Ca pretreatment of R. meliloti sustained further adsorption at low Ca levels and low pH substantially affected their ability to adsorb. Low pH appears to affect the stability of binding causing desorption of the previously bound bacteria. The presence of saturating concentrations of heterologous R. leguminosarum bv. trifolii A118, did not prevent the expression of divalent cations and pH requirements, as well as their interaction. Our results suggest that rhizobial binding to the root surface already shows the Ca and pH dependence of alfalfa nodulation, which was generally associated to some event prior to rhizobial penetration of root hairs.